More electronic devices have rapidly become portable and cordless these days, and the demand for smaller and lighter secondary batteries having greater energy density, which serve as power source to operate these devices, is increasing. Above all, great expectations are placed on a non-aqueous electrolyte secondary battery employing a negative electrode having lithium as an active material since it has high voltage and high energy density.
The above-mentioned battery utilizes, for example, a lithium-containing metal oxide for the positive electrode active material. Likewise, materials capable of absorbing and desorbing lithium, such as carbon material, are employed for the negative electrode.
Assuring security is one of the important issues in non-aqueous electrolyte secondary batteries. In particular, when a lithium ion secondary battery is overcharged due to breakdown of a charge control circuit, excessive lithium ions in the positive electrode are extracted and migrate to the negative electrode. Accordingly, more of the prescribed amount of lithium is absorbed in the negative electrode or is deposited on the negative electrode surface as metallic lithium. If it is forcibly kept charging in such a state, internal resistance in the battery will increase, resulting in the excessive heat generation.
In order to cope with the excessive heat generation, it is proposed to provide a positive temperature coefficient (PTC) thermistor or a temperature sensing type current breaking device such as fuse outside the battery. Use of the temperature sensing type current breaking device allows the electric current to be cut off without fail, thereby safety of the battery can be ensured. Likewise, Japanese Laid-open patent publication No. Hei 6-231749, Hei 10-125353 and Hei 10-241665 have suggested a method to equip a current breaking device having a positive temperature coefficient of resistance inside the battery. The specification of U.S. Pat. No. 4,943,497 further disclosed a means for cutting off the charge current by sensing a change in the internal pressure of the battery from a viewpoint of solving the problem of overcharging. Referring conventional current breaking devices, however, cutting costs is difficult and providing them inside the small and thin battery is structurally troublesome.
Consequently, Japanese Laid-open patent publication No. Hei 1-206571, Hei 6-338347 and Hei 7-302614 have suggested a method in which an additive undergoing a reversible oxidation reduction reaction is added to an electrolyte and electric energy fed into the inside of a battery is self-consumed by redox shuttle mechanism. When the overcharge current is increased, however, it can be hardly say that the battery employing the redox shuttle mechanism is safe because the redox reaction rate and the lithium ion moving rate have their limits.
At the same time, Japanese Laid-open patent publication No. Hei 9-50822, Hei 10-50342, Hei 9-106835, Hei 10-321258, Japanese Patent No. 2939469 and Japanese Laid-open patent publication No. 2000-58117 have suggested a means for adding aromatic compounds having a methoxy group and a halogen element, biphenyl, thiophene, terphenyl, aromatic ether and the like to the inside of a battery. These additives polymerize in the moderate overcharge process to cause temperature increase in the battery. As a result, micropores of its separators are closed to cut off the electric current.
In the battery having a temperature sensing type current breaking device outside thereof and the one having a current breaking device having a positive temperature coefficient of resistance inside thereof as mentioned above, the device itself is heated and the resistance of the device is increased to cut off the current when a large amount of current, which is 5 to 6 times (5 to 6 C) or greater than the battery capacity, flows during overcharge. Conversely, when they are overcharged at the normal charge and discharge current (1 to 2 C) of battery, safety cannot be fully ensured because there is not an adequate increase in temperature and resistance of the devices. However, use of a device in which resistance increases at a current of 1 to 2 C will impair battery performance.
When the battery having an electrolyte added with the aforementioned additive is overcharged at the normal current (1 to 2 C), polymerization of the additive on the electrodes and increase in electrode resistance are observed. Conversely, when it is overcharged at a large current of 5 to 6 C, safety cannot be fully ensured because the polymerization of the additive lags behind the charge.
In the case that an additive is added to the electrolyte in the battery having a temperature sensing type current breaking device outside the battery, safety is ensured when overcharged at a current of 1 to 2 C or a large current of 5 C or more. When overcharged at a current of 3 to 5 C, however, safety is not fully assured because the temperature sensing type current breaking device does not operate sensitively and the polymerization of the additive lags behind the charge.
In view of the aforementioned facts, an object of the present invention is to provide a battery wherein safety is ensured even when it is overcharged in any current range.